Fixed wavelength and tunable wavelength all-fiber Fabry Perot filters have been described in several U.S. patent applications including U.S. Pat. Nos. 5,212,746; 5,289,552; 5,212,745; 5,375,181; 5,422,970; 5,509,093; and 5,563,973. These patents provide descriptions of the manufacture of ferrule assemblies containing axial optical fibers as components of FFP filters comprising mirror-ended ferrules and embedded- mirror wafered ferrules. These patents also describe methods of making the mirror-ended ferrules and embedded-mirror wafered ferrules, methods of aligning the fibers of the ferrules to form an Fabry Perot optical cavity filter and ferrule alignment fixtures for use with such aligned ferrule assemblies. Alignment fixtures which facilitate alignment of fixed wavelength FFPs are described as well as alignment fixtures that facilitate wavelength tuning of the optical cavity of the filter without significant detriment to fiber alignment. Methods and alignment fixtures useful for passive and active temperature compensation and small range temperature tuning (or trimming) of fixed FFPs are also described in these patents.
U.S. patents and pending U.S. patent applications: 5,425,039; U.S. Pat. Ser. No. 08/984,245; now U.S. Pat. No. 6,044,189; U.S. Pat. Ser. No. 08/897,474; U.S. Pat. No. 5,892,582; U.S. Pat. Ser. No. 08/805,461; U.S. Pat. Ser. No. 08/833,602 U.S. Pat. No. 5,583,437; U.S. Pat. Ser. No. 60/028,517; U.S. Pat. Ser. No. 60/031,562; and U.S. Pat. Ser. No. 60/058,090 describe methods of using fixed and tunable FFP filters in wavelength scanners, wavelength references, spectrum analyzers and in optical fiber lasers. These patents are incorporated in their entirety by reference herein to the extent not inconsistent herewith.
For signal detection in dense WDM (50 GHz signal separation) applications using a filter with single path construction, a filter with a bandwidth of 0.02 nm (a finesse of 3000 for an FSR of 60 nm) is required. Filter requirements are less stringent if two filters cascaded in series are employed. In this case, a filter having a bandwidth of 0.12 nm (a finesse of 500 for an FSR of 60 nm) is required. The use of two or more filters in series would provide significant advantages for obtaining required signal resolution employing filters which have less stringent optical requirements and as such are less expensive and easier to manufacture. Such filters are also more reliably manufactured. However, cascading of filters in series generates other difficulties, particularly when the filters are wavelength tuned during operation, since tuning of the separate filters must be coordinated .
All-fiber Fabry Perot filters are very attractive candidates for high resolution, high finesse applications. Significant cost and efficiency of manufacture benefits could be achieved by the use of cascaded FFP configurations. This applications provides cascaded multiple fiber FP filters with significantly improved properties for high resolution and high finesse application. This application also provides alignment fixtures for use generally in any FFP filter that are easy to manufacture and less expensive to manufacture then current alignment fixtures. The fixtures disclosed herein are more particularly useful in tunable cascaded multiple fiber FFPS.
Current alignment fixtures for FFP can used end-brackets made of Kovar /stainless steel that require the use of ED machining which are relatively costly parts. The use of these precision end-brackets does not require any additional precision alignment equipment; however, several minutes to an hour may be required to adjust alignment of the FFP using such fixtures. Further, environmental testing (temperature cycling) usually yields only a 50% passage rate of FFPs. No angular adjustments can be made during assembly with such fixtures and small end-angle perpendicularity problems cannot be adjusted out. Loss performance of filters may be effected by the alignment techniques used with these endbracket fixtures. This invention provides an external alignment technique (using an alignment stage) and relatively low cost alignment fixtures that can lock and maintain the ferrule alignment obtained using the alignment stage.
All references cited in this application are incorporated in their entirety herein to the extent that their teaching are not inconsistent with those of the present application.